Irradiation of 1-(p-nitrophenyl)-5,5-dimethyl-1,3-hexadiyne (NDHD) in aqueous sulfuric acid yields conjugated allenyl ketones (1 and 2) and beta-dicarbonyl compounds (3 and 4). The products 1 and 2 are primary photoproducts while 3 and 4 are secondary products formed by thermal hydration of the primary photoproducts showing the maximum quantum yield at H-0 = -1.0. A mechanism involving a synchronous addition of hydronium ion (H3O+) to a triplet state of NDHD is proposed.